1. Field of the Art
The present invention relates in general to a method for coating a metal part with a synthetic resin material, and more particularly to improvements in the art of applying a resin layer to an outer surface of a metallic core member to produce a resin-coated metal part, by positioning the core member heated to an elevated temperature within a powdered mass of a thermally fusible resin.
2. Description of Related Art
Various resin-coated metal parts are known. FIG. 6 shows an example of such resin-coated metal parts in the form of a pair of lobe-type rotors 4 for a rotary fluid machine of a Roots type such as a supercharger used on an engine of an automotive vehicle to increase volumetric efficiency by forcing a greater quantity of air into the cylinders. The supercharger has a housing which consists of a hollow housing body 2, and a pair of end plates (not shown) which close opposite open ends of the hollow housing body 2, and cooperate with the hollow housing body 2 to define an air-tight pump chamber 3. The housing rotatably supports a pair of parallel support shafts 6, 6 which support the corresponding lobe-type rotors 4, 4 accommodated in the pump chamber 3. The two lobe-type rotors 4, 4 are coupled to each other by a pair of timing gears (not shown) fixed to one end of the corresponding support shafts 6, 6, so that the two rotors 4, 4 are rotated in opposite directions at the same angular velocity, whereby air is sucked into the pump chamber 3 through an inlet 8 formed in the housing body 2, and the compressed air is discharged from the pump chamber 3 through an outlet 10 also formed in the housing body 2.
Each lobe-type rotor 4, 4 consists of a metallic core member 11 and a resin layer 12 of a suitable thickness which covers an outer peripheral surface and opposite end faces of the core member 11. The resin layer 12 is applied to minimize gaps between the two rotors 4, 4, and between the rotors 4, 4 and the inner surface of the housing body 2, and to thereby improve the volumetric efficiency of the supercharger. The core member 11 consists of a pair of lobes, and has a transverse cross sectional shape similar to the shape of a cocoon or peanut shell.
For applying such a synthetic resin coating (hereinafter called "resin layer") to the outer surface of a metallic core member, the present applicants have attempted to practice a method wherein the metallic core member is heated to a temperature higher than a melting point of a thermally fusible synthetic resin while the core member is positioned within a powdered mass of the synthetic resin, so that a portion of the powdered mass surrounding the outer surface of the core member is melted and deposited on the outer surace of the core member. To this end, the core member is immersed into the powdered mass of the synthetic resin accommodated in a container. Alternatively, the core member is first placed within the container and the powdered mass of the synthetic resin is introduced into the container, so as to embed the core member in the powdered mass. Subsequently, the metallic core member is induction-heated to a temperature higher than the melting point of the synthetic resin, by energizing a heating coil which is disposed around or within the container.
The above coating method permits formation of a resin layer on the outer surface of the metallic core member in an efficient manner with relatively simple and less costly equipment. The formed resin layer has a which sufficient in actual practice degree of adhesion to the metallic core member.
However, the applicants found that the above method of forming the resin layer on the metallic core member embedded within a powdered mass of the synthetic resin, with a portion of the powdered mass adjacent to the core member being kept in a molten state, provides uneven results. More specifically, the above method tends to fail to provide a resin layer having a sufficient thickness on the lower end face of the core member. Further, the above method is likely to suffer from the creation of an air gap at the interface of the metallic core member and the formed resin layer, which may cause an unsatisfactory bond between the resin layer and the core member.
The above defects occur particularly easily where the resin material is introduced into a resin container in which the core member of the rotor has already been set in position. However, the same defects may also be encountered where the core member is introduced into the powdered resin mass which has already been introduced into the container.